Lithium secondary batteries have a wide range of application. Small-sized lithium secondary batteries are used as power sources for driving portable electronic communication equipments such as camcorders, mobile phones or notebook computer, in particular high-performance portable equipments, and now become dominant power supplies. Recently, medium and large-sized lithium secondary batteries of high output characteristics are developed for hybrid electric vehicles (HEV) or electric vehicles (EV) applications. The research and development of the medium and large-sized lithium secondary batteries moves toward environmentally friendly power sources for uninterruptible power supplies, motor tools, vessels, artificial satellites or military wireless telegraph sets and weapon systems in various application fields of industry inside and outside of the country including Japan, Europe and U.S.A.
Currently, high crystalline carbon such as natural graphite and artificial graphite, or low crystalline carbon such as non-graphitizable carbon and graphitizable carbon is used as an anode active material for a lithium secondary battery.
Natural graphite has advantages of low cost, a flat discharge curve in the negative voltage and excellent initial discharge capacity. However, the natural graphite has disadvantages of rapid reduction in charge/discharge efficiency and charge/discharge capacity when charge and discharge cycles are repeated.
Mesophase-based graphite has a shape of spherical granule, and has a high fill density to improve the energy density per volume of battery. And, the mesophase-based graphite is advantageous in molding a polarity plate. However, the mesophase-based graphite has a drawback of low reversible capacity.
Non-graphitizable carbon has advantages of excellent safety and large capacity. However, when compared with graphitic carbon, the non-graphitizable carbon has smaller particle size and micropores, leading to lower density. And, when the non-graphitizable carbon is pulverized, its particle shape and size becomes uniform. Thus, the non-graphitizable carbon has a low fill density, resulting in poor commercialization.
Recently, to meet the demand for safety and high capacity, lithium titanium oxide becomes the center of interest. The lithium titanium oxide is evaluated to be one of materials capable of improving safety and useful as an anode active material having a stable structure of spinel shape. The use of lithium titanium oxide as an anode active material results in has excellent durability as well as flatness of a voltage-discharge curve, excellent charge/discharge cycles, and improved power and high rate discharge characteristics. However, if lithium titanium oxide is used singularly, it has a reduction in battery characteristics due to its low average voltage.
Therefore, various methods have been suggested to solve the problems of the conventional anode active material. However, any anode active material was not yet discovered that is evaluated to be excellent in both of electrical characteristics and safety of a lithium secondary battery.
For example, Korean Patent Laid-open Publication No. 10-2004-0096279 discloses a method for preparing an anode active material with improved life and high rate discharge characteristics by doping graphite with a metal (or a nonmetal).
Korean Patent No. 10-0669335 discloses an anode for a lithium secondary battery, in which a thermosetting resin layer is formed on an anode current collector. Metal ions existing at the anode current collector diffuse into the thermosetting resin layer to create a concentration gradient. The surface roughness of an interface between the anode current collector and the thermosetting resin layer is increased to increase an adhesive strength therebetween, thereby improving life characteristics and safety of a battery.
Korean Patent Laid-open Publication No. 10-2008-0010944 discloses an anode for a lithium secondary battery, comprising an anode active material, and titanium oxide and styrene-butadiene(SBR) rubber on the surface of the anode active material. The used titanium oxide increases the surface resistance of carbon to prevent reduction of power in a battery. The styrene-butadiene rubber used together with titanium oxide promotes storage at high temperature due to excellent thermal safety and adhesive strength in itself.
Korean Patent No. 10-0861793 discloses an anode active material, in which graphite is surface-treated with TiO2 of high electroconductivity to improve high rate discharge characteristics of a lithium secondary battery.
However, techniques suggested in the above-mentioned prior arts are regarded as insufficient to improve safety while maintaining electrical characteristics of a lithium secondary battery.
Japanese Patent Laid-open Publication No. 10-241665 discloses an electrode manufactured by adding an active material, a conductive material, a binder and a PTC (Positive Temperature Coefficient) thermistor to an electrode slurry so as to improve safety of a lithium secondary battery.
Japanese Patent Laid-open Publication No. 2002-279996 discloses a non-aqueous secondary battery manufactured by thinly coating titanic acid and barium zirconate on any one of a cathode mix layer, an anode mix layer and a separator, or by adding titanic acid and barium zirconate in a non-aqueous electrolyte so as to improve high rate discharge characteristics of a lithium secondary battery.
However, the above-mentioned two prior arts disclose techniques for manufacturing an electrode by simply mixing or adding electrode materials, and thus it is improper to express that the prior arts relate to an anode active material. And, the techniques can improve safety of a battery to some extent, but may reduce electrical characteristics of a battery, and thus the prior arts are not evaluated to have sufficient effects.
Therefore, there are urgent demands for an anode active material for a lithium secondary battery that exhibits excellent safety while maintaining excellent battery characteristics, and a method for preparing the anode active material with excellent reproducibility and productivity.